Ah, if only life were like the pretty picture on the Public Prosecution Service's homepage. Shiny happy people of all ages, sizes and ethnicities frolic against a backdrop of shimmering light.

A black baby clutches an ice-cream cone; a man in a wheelchair is offered a bunch of flowers. And an elderly couple sit together on a park bench, enjoying the sunshine. The implication is that the PPS exists to protect this idyllic scenario; keep it safe from harm.

Meanwhile, back in the real world, two elderly people — David and Sarah Johnston, both in their late-80s — have just been put through an appalling ordeal by the PPS.

They were accused of a most repugnant crime: the killing of their severely-disabled grand-daughter, Rebecca McKeown. Rebecca died in 2001, five days after sustaining an injury alleged to have been the result of a sexual assault.

Every time I saw this frail, quiet, dignified couple walking into court, my mind did a double-take: it was difficult to imagine them being responsible for such a dreadful act against their vulnerable grand-daughter.

The Johnstons consistently denied the charges. Now they have walked free — completely exonerated — after a judge directed the jury to find them not guilty. The case collapsed in a mess of confusion over conflicting medical evidence and unanswered questions over whether a crime had been committed at all.

Tens of thousands of pounds of public money have been wasted. And to what end?

Two innocent old people have been unfairly accused of an obscene crime, forced to go through a deeply-traumatic experience from which they may never recover.

They have been publicly vilified, shunned by members of the community and intimidated out of their home of 40 years. And they still don't know how their grand-daughter met her untimely death.

The PPS said the collapse of the case was the result of developments “which could not have been foreseen”. Now I'm no lawyer, but why should the medical evidence have been such a surprise?

Why wasn't it more thoroughly scrutinised, probed for points of uncertainty, or confusion, before the enormous decision was taken to drag two bewildered pensioners into court?

In fact, the need for robust scrutiny was heightened further, given that this was a retrospective prosecution, taking place several years after the alleged event. Memories shift and change with time: this puts still-greater onus on prosecutors and police to be sure of their facts.

In this case, it appears as though the authorities became fixated on one extreme conclusion: that the grandparents were to blame. A sense of proportionality was lost and other potential areas of inquiry fell by the wayside.

So far, the PPS response to its failures in this case has been sketchy. It is not sufficient to put out a brief statement, sticking to the line that “the PPS is satisfied that this case was properly brought”.

At the time of writing, no spokesperson has been put forward to expand on this statement, or to answer legitimate questions about the handling of the case. That is a derisory way to respond to public interest in such a high-profile and costly failed trial.

It's far from the first time that the PPS has been accused of mistakes compounded by high-handedness. Penny Holloway, mother of the murdered schoolboy Thomas Devlin, spoke of the “spectacular, public and abysmally abject failure of the PPS to properly carry out its function”.

And a recent report by the Criminal Justice Inspection, looking at the PPS's engagement with the families of victims, found evidence of a “detectable disinclination” towards meeting relatives. A picture emerges of an obscure, complacent body, resistant to change. The accountability gap has also been raised at Stormont: as the situation stands, there is no one to answer on the floor of the Assembly on matters concerning the PPS.

Quite simply, it's not good enough for the PPS to hide behind a veil of self-satisfied, pompous statements. Such paternalistic arrogance is no longer acceptable. Independence is important, yes. But it must be balanced by transparency and accountability.

One final thought. The other big case currently in the headlines is the Mauritius murder trial. With the scenes of chaos there — the giggling in court, the jostling crowds — some may be tempted to characterise the Mauritian legal system as uncivilised and inept.

But with the shambles of the Rebecca McKeown trial, it's clear we can make no claims to superiority.